The long term objective of this research is to develop an affordable microwave radiometric system for non-invasive imaging of tissue temperatures.The aims of the Phase I research are as follows: 1) Evaluate the ability of a novel single-band microwave radiometer developed by us to discriminate between benign and malignant breast lesions that are detected during mammographic examinations. This radiometer, which measures average subsurface tissue temperatures, uses the latest microwave solid-state components as well as novel circuitry for which a patent has been applied for, to achieve high accuracy combined with low cost and small size. 2). Develop, build, and test an advanced version of the microwave radiometer described above. This more advanced radiometer would measure the microwave noise emitted from tissues at least eight different frequency bands, and would automatically convert the radiometric measurements to depth profiles of tissue temperatures. It would serve as a building block for the imaging radiometer to be developed in Phase II.